Actuatable passenger restraint systems for vehicles are well known in the art. One particular type of actuatable passenger restraint system includes an inflatable air bag mounted within the passenger compartment of the vehicle. The air bag has an associated, electrically actuatable ignitor, referred to as a squib. Such systems typically include a plurality of inertia sensing devices electrically connected in series with the squib and mounted at various locations in the vehicle for measuring the deceleration of the vehicle. When the inertia sensing devices are subjected to a crash force greater than a predetermined value, the inertia sensing devices each close an associated electrical switch causing an electric current of sufficient magnitude and duration to be passed through the squib to ignite the squib. The squib, when ignited, ignites a combustible gas generating composition and/or pierces a container of pressurized gas, which results in inflation of the air bag.
Many known inertia sensing devices used in actuatable passenger restraint systems are mechanical in nature. Such devices are typically mounted to the vehicle frame and include a pair of mechanically actuatable switch contacts and a resiliently biased weight. The weight is arranged such that when the vehicle decelerates, the weight physically moves relative to its mounting. The greater the amount and duration of the deceleration, the further the weight moves against the bias force. The switch contacts are mounted relative to the biased weight such that, when the weight moves a predetermined distance, the weight moves over or against the switch contacts causing them to close. When the switch contacts of each of the inertia sensors connected in series with the squib close, the squib is connected to a source of electrical energy sufficient to ignite the squib.
Still other known actuatable passenger restraint systems for vehicles include an electrical transducer or accelerometer for sensing vehicle deceleration. Such systems include a monitoring or evaluation circuit connected to the output of the transducer. The transducer provides an electrical signal having a value proportional to the vehicle's deceleration. If the crash sensing system includes only one accelerometer for the purpose of monitoring for a crash event, such a system is referred to in the art as a single point crash sensor system.
The monitoring circuit processes the transducer output signal. The processing of the accelerometer signal in a single point crash sensing system and the determination of whether a deployment crash event is occurring is the subject of several U.S. Patents. Known processing techniques include (i) integration of the acceleration signal to determine crash velocity, (ii) double integration of the acceleration signal to determined crash displacement, (iii) differentiation of the acceleration signal to determine crash jerk, (iv) frequency component monitoring to determine the presence of certain frequency components in the acceleration signal, or (v) determination of crash energy from the acceleration signal. Each of these techniques is referred to in the art as "the crash algorithm" or "the crash metrics." For any particular crash metric used, the determined value is typically compared against a predetermined threshold value. If the threshold value is exceeded or if certain values are determined, a deployment crash event is occurring.
The purpose of crash metrics is to distinguish between a deployment crash condition and a non-deployment crash condition. A non-deployment crash condition is one in which seat belts alone are sufficient to restrain the occupant and one in which deployment of the occupant's air bag will not enhance protection. A deployment crash condition is one in which deployment of the occupant's air bag will enhance protection for the occupant.
It is not desirable to inflate a vehicle air bag upon the occurrence of a non-deployment crash condition. Such needless deployment only increases the expense of repairing the vehicle after the crash event. A major problem that each of the crash metrics of the prior art is concerned with is the discrimination between a deployment crash event and a non-deployment crash event. By way of example, a 8-10 MPH zero degree crash into a barrier is considered a non-deployment crash event and a 14-17 MPH zero degree crash into a barrier is considered a deployment crash event. The actual values used are typically determined by the vehicle manufacturer. The margin between the two different crash events is relatively narrow. Also, other vehicle events occur that may result in an output from the accelerometer such as curb hits, undercarriage snags, etc., for which it is not desirable to deploy the air bag. The crash metrics must be capable of identifying such events as non-deployment events.
Also of concern is deployment timing. It is desirable to not only detect that a deployment crash event is occurring but to detect it early in the crash event so that the air bag deployment is timed to provide the maximum protection.